1. Field of the Invention
The present invention relates to the protection of a secret quantity or datum in an integrated circuit, for example, a smart card. “Secret datum” designates, in the sense of the present invention, any digital word representing any datum or address and which is desired to be protected against piracy.
2. Discussion of the Related Art
Many methods are known which enable encryption of data stored in memories external to a central processing unit, whether these memories are volatile or nonvolatile. Such methods make the data read directly from the memory by physical means (by an attack by means of electric sensors to pirate the datum) impossible to use. Such known methods suffer from a drawback, which is that the data encryption key must also be stored in a non-volatile memory (for example, an EEPROM or an OTP memory). This is a weak point of the system since this key may itself be obtained by physical attack.
To improve the security, it has already been provided to scramble the actual encryption key. However, the scrambling conditions are the same for all integrated circuit chips. It is thus possible, for a pirate, to obtain a key from a non-volatile memory of an authentic chip to copy it in chips industrially reproduced in an unauthorized manner, and to thus clone or imitate a series manufacturing, or to deduce encryption elements therefrom.